In the manufacture of pipe coatings, the pipe is heated to a high temperature and polymeric material applied as a powder or extruded on. The material is or becomes molten and conforms to the pipe surface. Usually, the pipe is spun or rotated about its axis. After sufficient time has elapsed for flow and/or curing to occur, the material is cooled to solidify it and to prevent damage during further handling. Damage can occur if the still molten coating comes into contact with equipment used to transport it such as supporting tires on a conveying line. In known processes cooling has been carried out by flooding the outside surface with cold water using many open or spray nozzle pipes. The process lasts until the material has reached the predetermined temperature.
With the known cooling procedures, it has always been a problem to obtain a defect free coating, especially with pipes that have raised weld profiles. It has been found that the difficulty arises in part due to shrinkage when the coating solidifies as well as in which order the different regions solidify.
Solidification of the outer surface first produces a skin layer which is highly stressed in tension and not yet bonded to the pipe surface. If the layer has a defect such as a pinhole or bubble, this becomes the weakest point and the coating can tear at this position. Where there is a concave curvature on the surface, such as at a neck area of a weld, the tension in the skin layer causes it to pull away from the pipe surface. The material at the pipe surface is still molten and yields, but at the same time creating pinholes and cavities to replace the displaced material. The cavities in the coating at the neck of the weld, which are referred to as tenting, can run for considerable distances along the weld length.
On a convex surface such as on the top of a weld, the still molten material under the frozen skin can be squeezed away to produce a lower than specified coating thickness when the coating becomes entirely frozen.